A conventional vacuum tube employs a hot cathode as an electron emitting source that requires pre-heating and therefore consumes a great deal of energy. The hot cathode is not suitable for miniaturization and integration by the employment of the semiconductor technology. Electron-emitting devices which utilize a cold cathode have attracted much attention as alternatives of the vacuum tube. Such devices include a field effect electron emitter element that emits electrons by a locally applied high electric field.
FIG. 7 is a sectional view that schematically shows a configuration of a field effect electron emitter element and FIGS. 8(A), 8(B), 8(C) and 8(D) are sectional views for explaining the processes used to manufacture the element of FIG. 7. As shown in FIG. 7, the prior art electron emitter element comprises a conical cold cathode 2 made of Mo formed on a silicon substrate 1, an opening 4 formed between the cold cathode 2, an insulation layer (silicon oxide layer) 3 formed on the substrate 1, and a control electrode 5 formed over the insulation layer 3 and extending partially above the opening 4. The cold cathode 2 and the control electrode 5, which constitute an electron emitter, are enclosed in an evacuated space by a sealing member not shown in the figure. Positive electric potential applied to the control electrode 5 concentrates a high intensity electric field on a sharp top of the cold cathode 2. The high intensity electric field modifies the work function of the cold cathode 2 and narrows a gap of the integrated potential across the boundary surface of the cold cathode 2, and electrons are emitted by field emission through a quantum mechanical tunnel process based on the Schottky effect.
The field effect electron emitter element provided with the conical cold cathode 2 is manufactured as follows. As shown in FIG. 8(A), the silicon oxide insulation layer 3 is formed on the semiconductor substrate 1 made for example of Si. The control electrode 5 is formed by patterning a Mo layer deposited on the insulation layer 3. The opening 4 is then formed by selective etching. As shown in FIG. 8(B), the substrate 1 is rotated around the X axis and an A1 layer 6 is formed on the control electrode 5 by vapor deposition so that an end face of the A1 layer 6 may tilt by a predetermined angle .theta.. Then, as FIG. 8(C) illustrates, Mo is deposited vertically onto the substrate 1 by the electron beam vapor deposition technique. Since Mo is deposited on a side surface of the A1 layer 6 as well as on the A1 layer 6 and the substrate 1 through the electron beam deposition process, a diameter of an opening on the A1 layer 6 and deposition area of a Mo layer on the substrate 1 decrease as the deposition of the Mo layer 7 proceeds. Thus, the conical cathode 2 is formed on the substrate 1. Finally, as shown in FIG. 8(D), the field effect electron emitter element provided with the conical cold cathode 2 and the control electrode 5 facing to the sharp top of the cathode 2 is obtained by removing the Mo layer 7 and the A1 layer 6.
The field effect electron emitter element described above is attractive in that the element does not need to have a pre-heating means and has a structure which is well suited for down-sizing. However, the production accuracy and reproducibility of area and height, and therefore the production yield, of the conical cathode 2 are not acceptable enough to practically use the device of the prior art because the conical cathode 2 is formed by gradually closing the opening by vapor deposition.
The prior art electron emitter element is not suitable to be employed in a large current capacity switching device because a current obtained at a top of the electron emitter (emitter current density) is small.
In view of the foregoing, an object of the present invention is to provide a switching apparatus that multiplies electrons emitted by field emission from a cold cathode and is suitable for switching a large current.